1. Field of the Invention
This invention relates to an internal combustion engine, and more particularly to a combustion state detection apparatus and a combustion state detection method for an internal combustion engine, wherein the occurrence of a preignition or the premonitory phenomenon thereof is detected on the basis of an ionic current involved in combustion.
2. Description of the Related Art
In an internal combustion engine of spark ignition type, a mixture sometimes self-fires earlier than a regular ignition timing, to incur preignition (earlier ignition). Since the durability of the internal combustion engine degrades due to the occurrence of the preignition, this preignition needs to be immediately suppressed at the occurrence thereof. Therefore, Patent Document 1 (Japanese Patent No. 3,552,142) has hitherto proposed a technique wherein the combustion state of the internal combustion engine is grasped from an ionic current flowing across the electrodes of an ignition plug, and the preignition is decided on the ground that the ionic current based on combustion appears before the fall of an ignition signal.
There has also been known a technique wherein the occurrence timing of that premonitory phenomenon (postignition) of the preignition which incurs self-firing immediately after ignition is measured on the basis of the ionic current flowing through the ignition plug.
However, there has been the problem that, in a case where “smoldering” (a phenomenon in which additives, etc. contained in fuel or lubricating oil carbonize to deposit carbon to the ignition plug) has occurred in the ignition plug, the preignition is erroneously detected. Therefore, a method wherein, in the case of the occurrence of the smoldering, the detection of the preignition is prohibited, and the same suppression control as at the occurrence of the preignition is performed, has been known from Patent Document 1.
The reason therefor is that, in the case of the occurrence of the smoldering in the ignition plug, the value of the insulation resistance between the ignition plug electrodes lowers, so a leak current flows in the same direction as that of the ionic current across the electrodes of the ignition plug before the fall of the ignition signal (in a primary current conduction period). The appearance period of the leak current tends to lengthen more as the degree of the smoldering becomes severer, and the appearance timing of the ionic current tends to become earlier as the strength of the preignition heightens more. The leak current and the ionic current based on the preignition overlap in some cases on account of such characteristics of both the currents. This poses the problem that the occurrence of the preignition cannot be decided simply in accordance with the existence or nonexistence of the ionic current.
Also in the premonitory phenomenon of the preignition, there is the problem that, in the case of the occurrence of the smoldering, the appearance position of the ionic current based on the combustion as is superposed on the leak current becomes difficult to be accurately grasped.